The invention relates to the general field of measurements effected with the aid of sensors of parameters such as the temperature of a fluid, for example.
The invention relates more particularly to correcting a measurement signal delivered by a temperature sensor.
The invention thus has a preferred but non-limiting application in the field of aviation and notably in the field of control systems for aircraft engines such as turbojet engines.
As is known, to regulate and adapt the control of a jet engine to various flight constraints, it is necessary to measure the temperature of the various streams of gas passing through the turbojet engine (referred to as stream temperatures). To this end, temperature sensors such as probes or thermocouples are used, positioned at various places in the gas stream channel.
Temperature sensors generally suffer from thermal inertia that is specific to each sensor and that depends in particular on the mass or the size of the sensor. This inertia is reflected in a time shift between the moment at which the measurement is effected by the sensor and the moment at which it delivers a signal in response to that measurement. This is referred to as the measurement lag effect and can cause malfunctions of the turbojet engine because of poor adaptation thereof, in particular during rapid variations in the temperatures of the gas streams.
To alleviate this problem there exist techniques for correcting the measurement signals delivered by a temperature sensor that compensate the lag effect induced by the inertia of the sensor. One such technique is described in U.S. Pat. No. 5,080,496, for example.
Those techniques generally rely on digital modeling of the inertia of the sensor using a filter with parameters set by estimating the time constant of the sensor. As is known in itself, the time constant of a measurement sensor characterizes its response time, i.e. its inertia.
Prior art techniques for estimating the time constant of a temperature sensor use fixed graphs depending on one or more parameters, for example the flow rate of the fluid in which the sensor is placed. Those graphs indicate mean values of time constants for response time templates and predetermined conditions. In other words, they do not in fact take account of the spread of inertia from one temperature sensor to another.
Current fabrication technologies do not enable temperature sensors for controlling turbojet engines to be produced at low cost and that also comply with a response time template subject to little spread.
Consequently, it is difficult to obtain graphs adapted to the various temperature sensors concerned. Numerous problems have arisen when the time constants of the sensors mounted in a turbojet engine depart considerably from the values given by these graphs.
One solution would be to test each temperature sensor, for example in a wind tunnel, to determine its time constant under predefined conditions, and to extrapolate the graphs as a function of the time constant determined in this way. Such a test is particularly costly, however, and represents approximately one-third of the price of the temperature sensor. Consequently, it cannot be used for each temperature sensor, which means that a temperature sensor outside an acceptance template for which a graph is available might not be detected.
Furthermore, such tests are often carried out at fluid flow rates limited by the capacities of the wind tunnel and are generally not able to cover the range of working flow rates in turbojet engine applications. Extrapolating graphs to cover all the range of working flow rates introduces inaccuracies into the acquisition system of the temperature sensor.
Moreover, as mentioned above, the time constant of a temperature sensor depends on parameters such as the flow rate of the fluid in which the sensor is placed. This means that in order to estimate the time constant of a temperature sensor it is necessary first to estimate this fluid flow rate. Consequently, it is necessary to use additional estimator modules on the turbojet engine, which makes correcting measurements even more complex.
Consequently, there is a need for a simple method of correcting measurement signals delivered by a temperature sensor that allows high-quality compensation of the lag effect introduced by the sensor, regardless of the time constant of the sensor.